Radiative antenna, e.g., dipole and monopole, have been increasingly used for ultrahigh field MRI ^1-5. Due to unique structures, the induced current elimination (ICE) approach is a suitable decoupling method for radiative arrays ^6,7. However, in ICE-decoupled radiative arrays, decoupling elements have obvious “shielding effect” which may causes a decrease in the transmit field in the certain area of imaging subjects. This ultimately results in dark spots in the MR images ⁸. In this study, we aim to investigate the effect of ICE decoupling elements and their position to the B₁ fields of radiative arrays.A series of simulations were performed with ANSYS HFSS and designer (Canonsburg, PA, USA). In simulation, eight monopole elements (yellow color, Fig. 1A) and eight decoupling elements (green color, Fig. 1A) were equally spaced along the surface of a cylindrical former (25 cm O.D.). Both the monopole elements and decoupling elements are made of 10-mm-wide copper tapes with a length of 25 cm. The distance between decoupling elements and the coil former (D_d) was varied from 0 cm to 3 cm with a step of 0.5 cm. D_d=0 means the non-optimized array and was used for the baseline comparison.

Simulation: Fig. 2 shows the simulated plots of reflection coefficient (S₁₁) of each monopole element and the transmission coefficients between adjacent elements (S₂₁), next adjacent elements (S₃₁) and opposite elements (S₅₁). Fig. 3 shows the simulated B₁⁺ filed of ICE-decoupled monopole arrays with different D_d. These arrays were excited in birdcage-like mode, applying 1 W power to each port, with a sequential 45 degree phase increment. For the non-optimized array, i.e., D_d=0 cm, the B₁⁺ field decreased obviously at the areas near decoupling elements, as shown in the white circles in Fig. 3. As the D_d increase, this B₁⁺ decrease become less obvious. It can be concluded from Fig. 3 that the dark spots in human images are mainly caused by the diminished B₁⁺ field at these areas.

Experiments: To validate the simulation results, ICE-decoupled monopole arrays without (D_d=0 cm) and with optimization (D_d=2.5 cm) were built for comparison, as shown in Figs. 4A and 4B . S₂₁ plots between adjacent elements of both arrays were shown in Figs. 4C and 4D. Similar to simulation results, the isolation between adjacent elements is about -15 dB for optimized array and about -25 dB for non-optimized array. The average loaded Q values of each monopole element of the optimized and non-optimized arrays are ~16 and ~33, respectively.

A healthy female volunteer was scanned subsequently with the non-optimized and optimized array (written informed consent). B₁⁺ profiles on the same human head were mapped with a Turbo FLASH sequence and show in Figs. 4E and 4F. As expected, for non-optimized ICE-decoupled monopole array, dark spots were found at the peripheral areas in both B₁⁺ profile and MR images, as shown in Figs. 4E and 4G. For optimized array, however, the B₁⁺ field and MR image were more homogeneous and no dark spots were observed (Figs. 4F and 4H). Figs. 4G and 4H show GRE images using the two arrays. The imaging acquisition parameters were as follows: flip angle=25 degree, TR/TE= 120/6 ms, FOV=250×250 mm², matrix=256×256, slice thickness=5 mm, bandwidth=260 Hz/pixel. It is worth noting that optimized array has a little higher central SNR and lower surface SNR compared with the non-optimized array. This can also be predicated from the Q value results.

The MR imaging and simulation results show that an optimized arrange of ICE decoupling elements can be found to minimize the perturbation of decoupling elements to the B₁ fields of radiative coil arrays, and consequently imaging quality can be improved. The optimized distance between decoupling elements and coil elements in this specific array configuration was set to 2.5 cm. Compared with the non-optimized ICE decoupled monopole array, the optimized array has more homogeneous transmit field and has no dark spots or signal cancellations in the MR images.